Provided by: libfabric-dev_1.17.0-3build2_amd64 
NAME
fi_mr - Memory region operations
fi_mr_reg / fi_mr_regv / fi_mr_regattr
Register local memory buffers for direct fabric access
fi_close
Deregister registered memory buffers.
fi_mr_desc
Return a local descriptor associated with a registered memory region
fi_mr_key
Return the remote key needed to access a registered memory region
fi_mr_raw_attr
Return raw memory region attributes.
fi_mr_map_raw
Converts a raw memory region key into a key that is usable for data transfer operations.
fi_mr_unmap_key
Releases a previously mapped raw memory region key.
fi_mr_bind
Associate a registered memory region with a completion counter or an endpoint.
fi_mr_refresh
Updates the memory pages associated with a memory region.
fi_mr_enable
Enables a memory region for use.
SYNOPSIS
#include <rdma/fi_domain.h>
int fi_mr_reg(struct fid_domain *domain, const void *buf, size_t len,
uint64_t access, uint64_t offset, uint64_t requested_key,
uint64_t flags, struct fid_mr **mr, void *context);
int fi_mr_regv(struct fid_domain *domain, const struct iovec * iov,
size_t count, uint64_t access, uint64_t offset, uint64_t requested_key,
uint64_t flags, struct fid_mr **mr, void *context);
int fi_mr_regattr(struct fid_domain *domain, const struct fi_mr_attr *attr,
uint64_t flags, struct fid_mr **mr);
int fi_close(struct fid *mr);
void * fi_mr_desc(struct fid_mr *mr);
uint64_t fi_mr_key(struct fid_mr *mr);
int fi_mr_raw_attr(struct fid_mr *mr, uint64_t *base_addr,
uint8_t *raw_key, size_t *key_size, uint64_t flags);
int fi_mr_map_raw(struct fid_domain *domain, uint64_t base_addr,
uint8_t *raw_key, size_t key_size, uint64_t *key, uint64_t flags);
int fi_mr_unmap_key(struct fid_domain *domain, uint64_t key);
int fi_mr_bind(struct fid_mr *mr, struct fid *bfid, uint64_t flags);
int fi_mr_refresh(struct fid_mr *mr, const struct iovec *iov,
size_t count, uint64_t flags);
int fi_mr_enable(struct fid_mr *mr);
ARGUMENTS
domain Resource domain
mr Memory region
bfid Fabric identifier of an associated resource.
context
User specified context associated with the memory region.
buf Memory buffer to register with the fabric hardware.
len Length of memory buffer to register. Must be > 0.
iov Vectored memory buffer.
count Count of vectored buffer entries.
access Memory access permissions associated with registration
offset Optional specified offset for accessing specified registered buffers. This parameter is reserved
for future use and must be 0.
requested_key
Requested remote key associated with registered buffers. Parameter is ignored if FI_MR_PROV_KEY
flag is set in the domain mr_mode bits.
attr Memory region attributes
flags Additional flags to apply to the operation.
DESCRIPTION
Registered memory regions associate memory buffers with permissions granted for access by fabric re‐
sources. A memory buffer must be registered with a resource domain before it can be used as the target
of a remote RMA or atomic data transfer. Additionally, a fabric provider may require that data buffers
be registered before being used in local transfers. Memory registration restrictions are controlled us‐
ing a separate set of mode bits, specified through the domain attributes (mr_mode field). Each mr_mode
bit requires that an application take specific steps in order to use memory buffers with libfabric inter‐
faces.
The following apply to memory registration.
Default Memory Registration
If no mr_mode bits are set, the default behaviors describe below are followed. Historically,
these defaults were collectively referred to as scalable memory registration. The default re‐
quirements are outlined below, followed by definitions of how each mr_mode bit alters the defini‐
tion.
Compatibility: For library versions 1.4 and earlier, this was indicated by setting mr_mode to FI_MR_SCAL‐
ABLE and the fi_info mode bit FI_LOCAL_MR to 0. FI_MR_SCALABLE and FI_LOCAL_MR were deprecated in lib‐
fabric version 1.5, though they are supported for backwards compatibility purposes.
For security, memory registration is required for data buffers that are accessed directly by a peer
process. For example, registration is required for RMA target buffers (read or written to), and those
accessed by atomic or collective operations.
By default, registration occurs on virtual address ranges. Because registration refers to address
ranges, rather than allocated data buffers, the address ranges do not need to map to data buffers allo‐
cated by the application at the time the registration call is made. That is, an application can register
any range of addresses in their virtual address space, whether or not those addresses are backed by phys‐
ical pages or have been allocated.
Note that physical pages must back addresses prior to the addresses being accessed as part of a data
transfer operation, or the data transfers will fail. Additionally, depending on the operation, this
could result in the local process receiving a segmentation fault for accessing invalid memory.
Once registered, the resulting memory regions are accessible by peers starting at a base address of 0.
That is, the target address that is specified is a byte offset into the registered region.
The application also selects the access key associated with the MR. The key size is restricted to a max‐
imum of 8 bytes.
With scalable registration, locally accessed data buffers are not registered. This includes source
buffers for all transmit operations – sends, tagged sends, RMA, and atomics – as well as buffers posted
for receive and tagged receive operations.
Although the default memory registration behavior is convenient for application developers, it is diffi‐
cult to implement in hardware. Attempts to hide the hardware requirements from the application often re‐
sults in significant and unacceptable impacts to performance. The following mr_mode bits are provided as
input into fi_getinfo. If a provider requires the behavior defined for an mr_mode bit, it will leave the
bit set on output to fi_getinfo. Otherwise, the provider can clear the bit to indicate that the behavior
is not needed.
By setting an mr_mode bit, the application has agreed to adjust its behavior as indicated. Importantly,
applications that choose to support an mr_mode must be prepared to handle the case where the mr_mode is
not required. A provider will clear an mr_mode bit if it is not needed.
FI_MR_LOCAL
When the FI_MR_LOCAL mode bit is set, applications must register all data buffers that will be ac‐
cessed by the local hardware and provide a valid desc parameter into applicable data transfer op‐
erations. When FI_MR_LOCAL is zero, applications are not required to register data buffers before
using them for local operations (e.g. send and receive data buffers). The desc parameter into da‐
ta transfer operations will be ignored in this case, unless otherwise required (e.g. se
FI_MR_HMEM). It is recommended that applications pass in NULL for desc when not required.
A provider may hide local registration requirements from applications by making use of an internal regis‐
tration cache or similar mechanisms. Such mechanisms, however, may negatively impact performance for
some applications, notably those which manage their own network buffers. In order to support as broad
range of applications as possible, without unduly affecting their performance, applications that wish to
manage their own local memory registrations may do so by using the memory registration calls.
Note: the FI_MR_LOCAL mr_mode bit replaces the FI_LOCAL_MR fi_info mode bit. When FI_MR_LOCAL is set,
FI_LOCAL_MR is ignored.
FI_MR_RAW
Raw memory regions are used to support providers with keys larger than 64-bits or require setup at
the peer. When the FI_MR_RAW bit is set, applications must use fi_mr_raw_attr() locally and
fi_mr_map_raw() at the peer before targeting a memory region as part of any data transfer request.
FI_MR_VIRT_ADDR
The FI_MR_VIRT_ADDR bit indicates that the provider references memory regions by virtual address,
rather than a 0-based offset. Peers that target memory regions registered with FI_MR_VIRT_ADDR
specify the destination memory buffer using the target’s virtual address, with any offset into the
region specified as virtual address + offset. Support of this bit typically implies that peers
must exchange addressing data prior to initiating any RMA or atomic operation.
FI_MR_ALLOCATED
When set, all registered memory regions must be backed by physical memory pages at the time the
registration call is made.
FI_MR_PROV_KEY
This memory region mode indicates that the provider does not support application requested MR
keys. MR keys are returned by the provider. Applications that support FI_MR_PROV_KEY can obtain
the provider key using fi_mr_key(), unless FI_MR_RAW is also set. The returned key should then be
exchanged with peers prior to initiating an RMA or atomic operation.
FI_MR_MMU_NOTIFY
FI_MR_MMU_NOTIFY is typically set by providers that support memory registration against memory re‐
gions that are not necessarily backed by allocated physical pages at the time the memory registra‐
tion occurs. (That is, FI_MR_ALLOCATED is typically 0). However, such providers require that ap‐
plications notify the provider prior to the MR being accessed as part of a data transfer opera‐
tion. This notification informs the provider that all necessary physical pages now back the re‐
gion. The notification is necessary for providers that cannot hook directly into the operating
system page tables or memory management unit. See fi_mr_refresh() for notification details.
FI_MR_RMA_EVENT
This mode bit indicates that the provider must configure memory regions that are associated with
RMA events prior to their use. This includes all memory regions that are associated with comple‐
tion counters. When set, applications must indicate if a memory region will be associated with a
completion counter as part of the region’s creation. This is done by passing in the FI_RMA_EVENT
flag to the memory registration call.
Such memory regions will be created in a disabled state and must be associated with all completion coun‐
ters prior to being enabled. To enable a memory region, the application must call fi_mr_enable(). After
calling fi_mr_enable(), no further resource bindings may be made to the memory region.
FI_MR_ENDPOINT
This mode bit indicates that the provider associates memory regions with endpoints rather than do‐
mains. Memory regions that are registered with the provider are created in a disabled state and
must be bound to an endpoint prior to being enabled. To bind the MR with an endpoint, the appli‐
cation must use fi_mr_bind(). To enable the memory region, the application must call fi_mr_en‐
able().
FI_MR_HMEM
This mode bit is associated with the FI_HMEM capability. If FI_MR_HMEM is set, the application
must register buffers that were allocated using a device call and provide a valid desc parameter
into applicable data transfer operations even if they are only used for local operations
(e.g. send and receive data buffers). Device memory must be registered using the fi_mr_regattr
call, with the iface and device fields filled out.
If FI_MR_HMEM is set, but FI_MR_LOCAL is unset, only device buffers must be registered when used locally.
In this case, the desc parameter passed into data transfer operations must either be valid or NULL. Sim‐
ilarly, if FI_MR_LOCAL is set, but FI_MR_HMEM is not, the desc parameter must either be valid or NULL.
FI_MR_COLLECTIVE
This bit is associated with the FI_COLLECTIVE capability. When set, the provider requires that
memory regions used in collection operations must explicitly be registered for use with collective
calls. This requires registering regions passed to collective calls using the FI_COLLECTIVE flag.
Basic Memory Registration
Basic memory registration was deprecated in libfabric version 1.5, but is supported for backwards
compatibility. Basic memory registration is indicated by setting mr_mode equal to FI_MR_BASIC.
FI_MR_BASIC must be set alone and not paired with mr_mode bits. Unlike other mr_mode bits, if
FI_MR_BASIC is set on input to fi_getinfo(), it will not be cleared by the provider. That is,
setting mr_mode equal to FI_MR_BASIC forces basic registration if the provider supports it.
The behavior of basic registration is equivalent to requiring the following mr_mode bits: FI_MR_VIRT_AD‐
DR, FI_MR_ALLOCATED, and FI_MR_PROV_KEY. Additionally, providers that support basic registration usually
require the (deprecated) fi_info mode bit FI_LOCAL_MR, which was incorporated into the FI_MR_LOCAL
mr_mode bit.
The registrations functions – fi_mr_reg, fi_mr_regv, and fi_mr_regattr – are used to register one or more
memory regions with fabric resources. The main difference between registration functions are the number
and type of parameters that they accept as input. Otherwise, they perform the same general function.
By default, memory registration completes synchronously. I.e. the registration call will not return un‐
til the registration has completed. Memory registration can complete asynchronous by binding the re‐
source domain to an event queue using the FI_REG_MR flag. See fi_domain_bind. When memory registration
is asynchronous, in order to avoid a race condition between the registration call returning and the cor‐
responding reading of the event from the EQ, the mr output parameter will be written before any event as‐
sociated with the operation may be read by the application. An asynchronous event will not be generated
unless the registration call returns success (0).
fi_mr_reg
The fi_mr_reg call registers the user-specified memory buffer with the resource domain. The buffer is
enabled for access by the fabric hardware based on the provided access permissions. See the access field
description for memory region attributes below.
Registered memory is associated with a local memory descriptor and, optionally, a remote memory key. A
memory descriptor is a provider specific identifier associated with registered memory. Memory descrip‐
tors often map to hardware specific indices or keys associated with the memory region. Remote memory
keys provide limited protection against unwanted access by a remote node. Remote accesses to a memory
region must provide the key associated with the registration.
Because MR keys must be provided by a remote process, an application can use the requested_key parameter
to indicate that a specific key value be returned. Support for user requested keys is provider specific
and is determined by the FI_MR_PROV_KEY flag value in the mr_mode domain attribute.
Remote RMA and atomic operations indicate the location within a registered memory region by specifying an
address. The location is referenced by adding the offset to either the base virtual address of the
buffer or to 0, depending on the mr_mode.
The offset parameter is reserved for future use and must be 0.
For asynchronous memory registration requests, the result will be reported to the user through an event
queue associated with the resource domain. If successful, the allocated memory region structure will be
returned to the user through the mr parameter. The mr address must remain valid until the registration
operation completes. The context specified with the registration request is returned with the completion
event.
fi_mr_regv
The fi_mr_regv call adds support for a scatter-gather list to fi_mr_reg. Multiple memory buffers are
registered as a single memory region. Otherwise, the operation is the same.
fi_mr_regattr
The fi_mr_regattr call is a more generic, extensible registration call that allows the user to specify
the registration request using a struct fi_mr_attr (defined below).
fi_close
Fi_close is used to release all resources associated with a registering a memory region. Once unregis‐
tered, further access to the registered memory is not guaranteed. Active or queued operations that ref‐
erence a memory region being closed may fail or result in accesses to invalid memory. Applications are
responsible for ensuring that a MR is no longer needed prior to closing it. Note that accesses to a
closed MR from a remote peer will result in an error at the peer. The state of the local endpoint will
be unaffected.
When closing the MR, there must be no opened endpoints or counters associated with the MR. If resources
are still associated with the MR when attempting to close, the call will return -FI_EBUSY.
fi_mr_desc
Obtains the local memory descriptor associated with a MR. The memory registration must have completed
successfully before invoking this call.
fi_mr_key
Returns the remote protection key associated with a MR. The memory registration must have completed suc‐
cessfully before invoking this. The returned key may be used in data transfer operations at a peer. If
the FI_RAW_MR mode bit has been set for the domain, then the memory key must be obtained using the
fi_mr_raw_key function instead. A return value of FI_KEY_NOTAVAIL will be returned if the registration
has not completed or a raw memory key is required.
fi_mr_raw_attr
Returns the raw, remote protection key and base address associated with a MR. The memory registration
must have completed successfully before invoking this routine. Use of this call is required if the
FI_RAW_MR mode bit has been set by the provider; however, it is safe to use this call with any memory re‐
gion.
On input, the key_size parameter should indicate the size of the raw_key buffer. If the actual key is
larger than what can fit into the buffer, it will return -FI_ETOOSMALL. On output, key_size is set to
the size of the buffer needed to store the key, which may be larger than the input value. The needed
key_size can also be obtained through the mr_key_size domain attribute (fi_domain_attr) field.
A raw key must be mapped by a peer before it can be used in data transfer operations. See fi_mr_map_raw
below.
fi_mr_map_raw
Raw protection keys must be mapped to a usable key value before they can be used for data transfer opera‐
tions. The mapping is done by the peer that initiates the RMA or atomic operation. The mapping function
takes as input the raw key and its size, and returns the mapped key. Use of the fi_mr_map_raw function
is required if the peer has the FI_RAW_MR mode bit set, but this routine may be called on any valid key.
All mapped keys must be freed by calling fi_mr_unmap_key when access to the peer memory region is no
longer necessary.
fi_mr_unmap_key
This call releases any resources that may have been allocated as part of mapping a raw memory key. All
mapped keys must be freed before the corresponding domain is closed.
fi_mr_bind
The fi_mr_bind function associates a memory region with a counter or endpoint. Counter bindings are
needed by providers that support the generation of completions based on fabric operations. Endpoint
bindings are needed if the provider associates memory regions with endpoints (see FI_MR_ENDPOINT).
When binding with a counter, the type of events tracked against the memory region is based on the bitwise
OR of the following flags.
FI_REMOTE_WRITE
Generates an event whenever a remote RMA write or atomic operation modifies the memory region.
Use of this flag requires that the endpoint through which the MR is accessed be created with the
FI_RMA_EVENT capability.
When binding the memory region to an endpoint, flags should be 0.
fi_mr_refresh
The use of this call is required to notify the provider of any change to the physical pages backing a
registered memory region if the FI_MR_MMU_NOTIFY mode bit has been set. This call informs the provider
that the page table entries associated with the region may have been modified, and the provider should
verify and update the registered region accordingly. The iov parameter is optional and may be used to
specify which portions of the registered region requires updating. Providers are only guaranteed to up‐
date the specified address ranges.
The refresh operation has the effect of disabling and re-enabling access to the registered region. Any
operations from peers that attempt to access the region will fail while the refresh is occurring. Addi‐
tionally, attempts to access the region by the local process through libfabric APIs may result in a page
fault or other fatal operation.
The fi_mr_refresh call is only needed if the physical pages might have been updated after the memory re‐
gion was created.
fi_mr_enable
The enable call is used with memory registration associated with the FI_MR_RMA_EVENT mode bit. Memory
regions created in the disabled state must be explicitly enabled after being fully configured by the ap‐
plication. Any resource bindings to the MR must be done prior to enabling the MR.
MEMORY REGION ATTRIBUTES
Memory regions are created using the following attributes. The struct fi_mr_attr is passed into
fi_mr_regattr, but individual fields also apply to other memory registration calls, with the fields
passed directly into calls as function parameters.
struct fi_mr_attr {
const struct iovec *mr_iov;
size_t iov_count;
uint64_t access;
uint64_t offset;
uint64_t requested_key;
void *context;
size_t auth_key_size;
uint8_t *auth_key;
enum fi_hmem_iface iface;
union {
uint64_t reserved;
int cuda;
int ze
} device;
};
mr_iov
This is an IO vector of addresses that will represent a single memory region. The number of entries in
the iovec is specified by iov_count.
iov_count
The number of entries in the mr_iov array. The maximum number of memory buffers that may be associated
with a single memory region is specified as the mr_iov_limit domain attribute. See fi_domain(3).
access
Indicates the type of operations that the local or a peer endpoint may perform on registered memory re‐
gion. Supported access permissions are the bitwise OR of the following flags:
FI_SEND
The memory buffer may be used in outgoing message data transfers. This includes fi_msg and
fi_tagged send operations, as well as fi_collective operations.
FI_RECV
The memory buffer may be used to receive inbound message transfers. This includes fi_msg and
fi_tagged receive operations, as well as fi_collective operations.
FI_READ
The memory buffer may be used as the result buffer for RMA read and atomic operations on the ini‐
tiator side. Note that from the viewpoint of the application, the memory buffer is being written
into by the network.
FI_WRITE
The memory buffer may be used as the source buffer for RMA write and atomic operations on the ini‐
tiator side. Note that from the viewpoint of the application, the endpoint is reading from the
memory buffer and copying the data onto the network.
FI_REMOTE_READ
The memory buffer may be used as the source buffer of an RMA read operation on the target side.
The contents of the memory buffer are not modified by such operations.
FI_REMOTE_WRITE
The memory buffer may be used as the target buffer of an RMA write or atomic operation. The con‐
tents of the memory buffer may be modified as a result of such operations.
FI_COLLECTIVE
This flag provides an explicit indication that the memory buffer may be used with collective oper‐
ations. Use of this flag is required if the FI_MR_COLLECTIVE mr_mode bit has been set on the do‐
main. This flag should be paired with FI_SEND and/or FI_RECV
Note that some providers may not enforce fine grained access permissions. For example, a memory region
registered for FI_WRITE access may also behave as if FI_SEND were specified as well. Relaxed enforcement
of such access is permitted, though not guaranteed, provided security is maintained.
offset
The offset field is reserved for future use and must be 0.
requested_key
An application specified access key associated with the memory region. The MR key must be provided by a
remote process when performing RMA or atomic operations to a memory region. Applications can use the re‐
quested_key field to indicate that a specific key be used by the provider. This allows applications to
use well known key values, which can avoid applications needing to exchange and store keys. Support for
user requested keys is provider specific and is determined by the the FI_MR_PROV_KEY flag in the mr_mode
domain attribute field.
context
Application context associated with asynchronous memory registration operations. This value is returned
as part of any asynchronous event associated with the registration. This field is ignored for synchro‐
nous registration calls.
auth_key_size
The size of key referenced by the auth_key field in bytes, or 0 if no authorization key is given. This
field is ignored unless the fabric is opened with API version 1.5 or greater.
auth_key
Indicates the key to associate with this memory registration. Authorization keys are used to limit com‐
munication between endpoints. Only peer endpoints that are programmed to use the same authorization key
may access the memory region. The domain authorization key will be used if the auth_key_size provided is
0. This field is ignored unless the fabric is opened with API version 1.5 or greater.
iface
Indicates the software interfaces used by the application to allocate and manage the memory region. This
field is ignored unless the application has requested the FI_HMEM capability.
FI_HMEM_SYSTEM
Uses standard operating system calls and libraries, such as malloc, calloc, realloc, mmap, and
free.
FI_HMEM_CUDA
Uses Nvidia CUDA interfaces such as cuMemAlloc, cuMemAllocHost, cuMemAllocManaged, cuMemFree, cud‐
aMalloc, cudaFree.
FI_HMEM_ROCR
Uses AMD ROCR interfaces such as hsa_memory_allocate and hsa_memory_free.
FI_HMEM_ZE
Uses oneAPI Level Zero interfaces such as zeDriverAllocSharedMem, zeDriverFreeMem.
FI_HMEM_NEURON
Uses the AWS Neuron SDK to support AWS Trainium devices.
FI_HMEM_SYNAPSEAI
Uses the SynapseAI API to support Habana Gaudi devices.
device
Reserved 64 bits for device identifier if using non-standard HMEM interface. This field is ignore unless
the iface field is valid.
cuda For FI_HMEM_CUDA, this is equivalent to CUdevice (int).
ze For FI_HMEM_ZE, this is equivalent to the ze_device_handle_t index (int).
neuron For FI_HMEM_NEURON, the device identifier for AWS Trainium devices.
synapseai
For FI_HMEM_SYNAPSEAI, the device identifier for Habana Gaudi hardware.
NOTES
Direct access to an application’s memory by a remote peer requires that the application register the tar‐
geted memory buffer(s). This is typically done by calling one of the fi_mr_reg* routines. For
FI_MR_PROV_KEY, the provider will return a key that must be used by the peer when accessing the memory
region. The application is responsible for transferring this key to the peer. If FI_MR_RAW mode has
been set, the key must be retrieved using the fi_mr_raw_attr function.
FI_RAW_MR allows support for providers that require more than 8-bytes for their protection keys or need
additional setup before a key can be used for transfers. After a raw key has been retrieved, it must be
exchanged with the remote peer. The peer must use fi_mr_map_raw to convert the raw key into a usable
64-bit key. The mapping must be done even if the raw key is 64-bits or smaller.
The raw key support functions are usable with all registered memory regions, even if FI_MR_RAW has not
been set. It is recommended that portable applications target using those interfaces; however, their use
does carry extra message and memory footprint overhead, making it less desirable for highly scalable
apps.
There may be cases where device peer to peer support should not be used or cannot be used, such as when
the PCIe ACS configuration does not permit the transfer. The FI_HMEM_DISABLE_P2P environment variable
can be set to notify Libfabric that peer to peer transactions should not be used. The provider may
choose to perform a copy instead, or will fail support for FI_HMEM if the provider is unable to do that.
FLAGS
The follow flag may be specified to any memory registration call.
FI_RMA_EVENT
This flag indicates that the specified memory region will be associated with a completion counter
used to count RMA operations that access the MR.
FI_RMA_PMEM
This flag indicates that the underlying memory region is backed by persistent memory and will be
used in RMA operations. It must be specified if persistent completion semantics or persistent da‐
ta transfers are required when accessing the registered region.
FI_HMEM_DEVICE_ONLY
This flag indicates that the memory is only accessible by a device. Which device is specified by
the fi_mr_attr fields iface and device. This refers to memory regions that were allocated using a
device API AllocDevice call (as opposed to using the host allocation or unified/shared memory al‐
location).
FI_HMEM_HOST_ALLOC
This flag indicates that the memory is owned by the host only. Whether it can be accessed by the
device is implementation dependent. The fi_mr_attr field iface is still used to identify the de‐
vice API, but the field device is ignored. This refers to memory regions that were allocated us‐
ing a device API AllocHost call (as opposed to using malloc-like host allocation, unified/shared
memory allocation, or AllocDevice).
MEMORY DOMAINS
Memory domains identify the physical separation of memory which may or may not be accessible through the
same virtual address space. Traditionally, applications only dealt with a single memory domain, that of
host memory tightly coupled with the system CPUs. With the introduction of device and non-uniform memory
subsystems, applications often need to be aware of which memory domain a particular virtual address maps
to.
As a general rule, separate physical devices can be considered to have their own memory domains. For ex‐
ample, a NIC may have user accessible memory, and would be considered a separate memory domain from memo‐
ry on a GPU. Both the NIC and GPU memory domains are separate from host system memory. Individual GPUs
or computation accelerators may have distinct memory domains, or may be connected in such a way (e.g. a
GPU specific fabric) that all GPUs would belong to the same memory domain. Unfortunately, identifying
memory domains is specific to each system and its physical and/or virtual configuration.
Understanding memory domains in heterogenous memory environments is important as it can impact data or‐
dering and visibility as viewed by an application. It is also important to understand which memory do‐
main an application is most tightly coupled to. In most cases, applications are tightly coupled to host
memory. However, an application running directly on a GPU or NIC may be more tightly coupled to memory
associated with those devices.
Memory regions are often associated with a single memory domain. The domain is often indicated by the
fi_mr_attr iface and device fields. Though it is possible for physical pages backing a virtual memory
region to migrate between memory domains based on access patterns. For example, the physical pages ref‐
erenced by a virtual address range could migrate between host memory and GPU memory, depending on which
computational unit is actively using it.
See the fi_endpoint(3) and fi_cq(3) man pages for addition discussion on message, data, and completion
ordering semantics, including the impact of memory domains.
RETURN VALUES
Returns 0 on success. On error, a negative value corresponding to fabric errno is returned.
Fabric errno values are defined in rdma/fi_errno.h.
ERRORS
-FI_ENOKEY
The requested_key is already in use.
-FI_EKEYREJECTED
The requested_key is not available. They key may be out of the range supported by the provider,
or the provider may not support user-requested memory registration keys.
-FI_ENOSYS
Returned by fi_mr_bind if the provider does not support reporting events based on access to regis‐
tered memory regions.
-FI_EBADFLAGS
Returned if the specified flags are not supported by the provider.
MEMORY REGISTRATION CACHE
Many hardware NICs accessed by libfabric require that data buffers be registered with the hardware while
the hardware accesses it. This ensures that the virtual to physical address mappings for those buffers
do not change while the transfer is occurring. The performance impact of registering memory regions can
be significant. As a result, some providers make use of a registration cache, particularly when working
with applications that are unable to manage their own network buffers. A registration cache avoids the
overhead of registering and unregistering a data buffer with each transfer.
If a registration cache is going to be used for host and device memory, the device must support unified
virtual addressing. If the device does not support unified virtual addressing, either an additional reg‐
istration cache is required to track this device memory, or device memory cannot be cached.
As a general rule, if hardware requires the FI_MR_LOCAL mode bit described above, but this is not sup‐
ported by the application, a memory registration cache may be in use. The following environment vari‐
ables may be used to configure registration caches.
FI_MR_CACHE_MAX_SIZE
This defines the total number of bytes for all memory regions that may be tracked by the cache.
If not set, the cache has no limit on how many bytes may be registered and cached. Setting this
will reduce the amount of memory that is not actively being used as part of a data transfer that
is registered with a provider. By default, the cache size is unlimited.
FI_MR_CACHE_MAX_COUNT
This defines the total number of memory regions that may be registered with the cache. If not
set, a default limit is chosen. Setting this will reduce the number of regions that are regis‐
tered, regardless of their size, which are not actively being used as part of a data transfer.
Setting this to zero will disable registration caching.
FI_MR_CACHE_MONITOR
The cache monitor is responsible for detecting system memory (FI_HMEM_SYSTEM) changes made between
the virtual addresses used by an application and the underlying physical pages. Valid monitor op‐
tions are: userfaultfd, memhooks, and disabled. Selecting disabled will turn off the registration
cache. Userfaultfd is a Linux kernel feature used to report virtual to physical address mapping
changes to user space. Memhooks operates by intercepting relevant memory allocation and dealloca‐
tion calls which may result in the mappings changing, such as malloc, mmap, free, etc. Note that
memhooks operates at the elf linker layer, and does not use glibc memory hooks.
FI_MR_CUDA_CACHE_MONITOR_ENABLED
The CUDA cache monitor is responsible for detecting CUDA device memory (FI_HMEM_CUDA) changes made
between the device virtual addresses used by an application and the underlying device physical
pages. Valid monitor options are: 0 or 1. Note that the CUDA memory monitor requires a CUDA
toolkit version with unified virtual addressing enabled.
FI_MR_ROCR_CACHE_MONITOR_ENABLED
The ROCR cache monitor is responsible for detecting ROCR device memory (FI_HMEM_ROCR) changes made
between the device virtual addresses used by an application and the underlying device physical
pages. Valid monitor options are: 0 or 1. Note that the ROCR memory monitor requires a ROCR ver‐
sion with unified virtual addressing enabled.
FI_MR_ZE_CACHE_MONITOR_ENABLED
The ZE cache monitor is responsible for detecting oneAPI Level Zero device memory (FI_HMEM_ZE)
changes made between the device virtual addresses used by an application and the underlying device
physical pages. Valid monitor options are: 0 or 1.
More direct access to the internal registration cache is possible through the fi_open() call, using the
“mr_cache” service name. Once opened, custom memory monitors may be installed. A memory monitor is a
component of the cache responsible for detecting changes in virtual to physical address mappings. Some
level of control over the cache is possible through the above mentioned environment variables.
SEE ALSO
fi_getinfo(3), fi_endpoint(3), fi_domain(3), fi_rma(3), fi_msg(3), fi_atomic(3)
AUTHORS
OpenFabrics.
Libfabric Programmer’s Manual 2022-12-11 fi_mr(3)